 |
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
|
| If you can't view the Datasheet, Please click here to try to view without PDF Reader . |
|
|
| Datasheet File OCR Text: |
| rev. 1.7 3/15 copyright ? 2015 by silicon laboratories AN607 AN607 si70 xx h umidity and t emperature s ensor d esigner ? s g uide 1. introduction this designer's guide describes the si 70xx family of humidity and temperature sensor products in a variety of different applications. the first half provides a detailed description of the si70xx family of humidity sensors including specific applicati on examples. system-level design consider ations including sens or placement, system calibration and product use are discussed. finally, special handling considerations for both the sensor device and the end product containing the sensor are discussed. the second half begins with a brief desc ription of humidity and the various ways it is quantified. it is important to understand the terminology and the relative merits of each measurement. next, methods for measuring humidity and measurement challenges are presented. finally , the impact of humidity on comfort is discussed. ? appendix a?industry specifications and guidelines? contains valuable reference information about industry standards applicable to the measurement and control of humidity. ? appendix b?equations for vapor pressure and humidity calculations? contains a description of several equations useful for humidity related calculations. ? appendix c?term, unit, and coefficient reference? cont ains a convenient reference containing unit conversions, coefficients and a glossary of humidi ty related terms used in this document . ? appendix d?nonlinear correction of voltage inputs with the si7013? explains the nonlinear correction of the si7013 voltage input. ? appendix e? thermal model for a sensor on a paddle? presents a thermal model for a sensor on a paddle. figure 1. si7013 relative humidity and temperature sensor shown with and without hydrophobic cover installed figure 2. si7053/4/5 temperature sensor figure 3. si7034 relative humidity and temperature sensor
AN607 2 rev. 1.7 2. si70xx monolithic humidi ty and temperature sensors the si70xx family uses state-of-the-art sensing technologi es to create precise monolithic humidity and temperature sensors. temperature is sensed by a precision vbe refere nced circuit on the die. humidity is sensed by measuring the capacitance change of a low-k dielectric layer applied to the surface of the die. co nsequently, both temperature and humidity are precisely measured in extremely close proximity on the same monolithic device, which is critical for accurate dew point measurement. the si7005 was the first-generation sensor in a 4 mm x 4 mm package. the si7015 is a pin-compatible upgrade (see ?an764: upgrading from the si7005 to the si7015? ). the si7006, si7007, si701 3, si7020, si7021, si7022, and si7023 second-generation parts come in an industry -standard 3 mm x 3 mm package. the si7034 is available in an industry standard 2x2 mm package. the si7006, si7013, si7020, si7021, and si7034 are i 2 c parts that all have similar block diagrams. for example, figure 4 shows a functional block diagram of the si7013 humidity and temperature sensor. very few external components are required. communication with the device is via the i 2 c bus sda and scl pins. the si7013 has an optional analog input path for measurement of a remote th ermistor or any other analog voltage. it also has a dual function pin for i 2 c address selection and thermistor biasing. d epending on the microcontroller used in the application, the 10 k ? pull-ups on the i 2 c bus may be included in the microcontroller. the only other component required is the 0.1 f power supply bypass capacitor. figure 4. si7013 functional block diagram the si7050/3/4/5 temperature sensors are functionally identical to the i 2 c temperature and humidity sensors, but there is no hole in the package, and only the temperature sensor functionality is supported. refer to the data sheets (e.g., si7013) for detailed pi n descriptions, register summary, and timing details. the si7007, si7022 and si7023 parts contain pulse width mo dulated (pwm) outputs. on one pin, the pulse width is proportional to temperature while the pulse width on a differen t pin is proportional to humidity. there is a select pin that can be used to interchange the assignment of the out puts pins. in most applications the pwm output of these parts is filtered to give a voltage proportional to temperature or humidity. the functional block for all the pwm output parts is the same, and this is shown in figure 5 for the si7022. adc humidity sensor temp sensor analog input digital logic i 2 c interface si7013 vddd vdda gndd gnda tgnd (exposed pad) sda scl vsns ad0/vout vinp vinn 3 4 89 1 10 2 5 6 7 1.25v ref AN607 rev. 1.7 3 figure 5. si7022 functional block diagram adc gnd humidity sensor control logic si7022 temp sensor 1.25v ref pwm interface pwm1 vdd calibration memory select pwm2 AN607 4 rev. 1.7 2.1. humidity sensor technology figure 6 illustrates the humidity sensor configuration. the dielectric layer is exposed to ambient air through the package opening. humidity in the air is exchanged with th e dielectric material until equilibrium is reached. the permittivity of the dielectric material is proportional to the amount of moisture it has absorbed. the capacitance increases as the ambient air becomes more humid and decr eases as the ambient air bec omes less humid. if the si70xx is used at the dew point and condensation occurs on the sensor, the on-chip heater can be activated to dry the sensor and restore operation once the sensor is above the dew point. figure 6. humidity sensor configuration 2.2. temperature sensor the si70xx parts have a very accurate (as good as 0.3 c over the full ?40 to +125 c temperature range) temperature sensor integrated into the device. the si70xx family of devices is desi gned to consume very little power and does not artificially heat the temperature sensor unless, of course, the heater is turned on. the thermal paddle on the bottom of the chip is el ectrically connected to ground inside the chip. the paddle can be soldered to a ground plane on the pcb which can in turn be extended bey ond the chip or can be left floating. in either case, the ground pad is the thermal input to the on-chip temperature sensor. care must be taken to isolate the chip from thermal sources in the system in which it is included. refer to the section on pcb layout guidelines for more details on pcb layout and system design consider ations to maximize sensor performance. digital output er increases as moisture is absorbed a/d polimide ? r moisture from humid air AN607 rev. 1.7 5 3. system design considerations 3.1. power requirements the si70xx devices are inherently low power if the heater is not used. for example, si7013 power consumption during a conversion is 150 a typical, and the power consum ption in idle mode is less than 100 na. this low power consumption means that there is no significant self-h eating in the conversion process. because the si7007, si7022, and si7023 pwm output parts do humidity and te mperature conversions twice a second to update the pwm output the power consumption if these parts is high er at 180 a typical. generally this still does not cause significant heating. 3.1.1. battery life considerations normally, battery life is rated in milliamp -hours (mah). for example, an energizer ? e91 battery is rated for approximately 2500 mah for light loads when allowed to discharge to 0.9 v. considering the case of an si7013, which has a v dd range of 1.71 to 3.6 v, two aa batteries in series provide 3.0 v when charged and 1.8 v when fully depleted, so this is acceptable. again, for the case of an si7013, a humidity and temp erature conversion typically consumes 150 a. in normal mode, the total time for an rh and temperature conversion is 8.4 msec, and, in fast mode, this is reduced to 5.5 msec. a temperature conversion is performed every time an rh conversion is done (to allow for temperature correction). if a conversion is performed once per second, then 31.5x10 6 conversions occur per y ear. the number of milliamp- hours is as follows: plugging in values, this translates to 11.1 ma-hours for one year in normal mode and 7.2 ma-hours for fast mode. the 100 na quiescent current of the si7013 will consume an additional nu mber of milliamp hour s, as shown below: at 100 na quiescent current, this is 0.876 ma-hours per year. even when targeting a 10-year battery life, the sensor itself would consume less than 120 ma hours of charge in normal mode. this means that only about 5% of the available 2500 mah is consumed by the sensor. of course, in a practical system, there are many other drains on a battery, and, often, coin cells, which have much less capacity than aa cells, are used. on the other hand, it is generally acceptable to reduce the conversion rate well below once per second, meaning th at a sensor, such as t he si7013, will generally co nsume a small portion of the available battery life. milliamp hours (mah) number of conversions conversion time (msec) ? conversion current (ma) 1000msec/sec 3600sec/hour ? ?? ------------------------------------------------------------------------------------- - ? = milliamp hours (mah) quiescent current (na) 8760 (hours/year) 10 6 na/ma ---------------------------------------------- - ? = AN607 6 rev. 1.7 3.2. temperature and humidity sensor placement the following are general guidelines for sensor placement: ?? for accurate humidity and temperature measurement, keep heating from other components to less than 0.2 c, or compensate for heating. ?? for rapid response to air temperature changes, keep th e thermal mass that is attached to the sensor low, and insulate the sensor from larger system thermal masses. ?? for rapid response to humidity changes, keep the sensor exposed to the ambient, or make the opening comparable in size to the cavity in which the sensor resides. ?? do not use materials, such as soft plastic, glue, or wood, in the vici nity of the humidity sensor since these can absorb or emit moisture as well as give off fumes that affect the senor reading. ?? protect the sensor against liquids and dust using the factory-installed eptfe filter or similar cover. ?? protect against esd with exposed ground metal. for the humidity sensors, if the sensor is not in a cavity, use the eptfe filter cover to prevent esd directly into the sensor area. use conformal coating material on the leads, or use esd diodes on all pins. unused pins may be connected to vdd for esd protection. for situations in which it is not possible to completely insulate the se nsor for the system, insulate as much as possible and use a temperature sensor connected to the system to allow compensation of residual heating. if a thermal model can be developed for the overall system, it is also possible to apply an inverse filter and speed up the response of the overall system to changes in t he ambient. these are discussed in more detail below. 3.2.1. place the sensor away from heat sources as air is heated or cooled, humidity will be reduced or increased by appr oximately 5% per degree. that is, increasing temperature by 1 c will reduce the relative humidity of 100% humid air to 95%rh or 50% humid air to 47.5% rh. this ?rule of thumb? is useful for esti mating the effect of small temperature increases. the first consideration in trying to measure humidity or te mperature outside of an encl osure is to place the sensor away from any heat sources internal to the enclosure or to thermally insulate the sensor from the internal heat source so it is better connected to the ambient envir onment than internal heat sources. for accurate humidity measurement, heating from other sources should be limited to no more than 0.2 c. in some cases, where the amount of heating is known or can be measured, it is possible to compensate for the heating or cooling. for larger temperature increases, the ?magnus? equation can be used to more accurately calculate the change of humidity for a chang e in temperature (see also appendix b). 3.2.2. thermal mass and thermal resistance when the ic is soldered down, it becomes thermally connected to the printed circuit board on which it is mounted. the printed circuit board is, in turn, thermally connected to the system it is mounted within. the time constant to respond to changes in ambient air temperature depends on the effective thermal mass the device is connected to as well as the effective thermal resistance. if the entire system (such as a thermostat) can be expected to go up or down in temper ature along with the ambient it is measuring, then separating th e sensor from the system is not necessa ry, but response time will generally be slow. a general model for this is shown in figure 7. AN607 rev. 1.7 7 figure 7. general thermal model for sensor placement if the system has a large thermal mass (c1) or other inte rnal heat sources the thermal resistance from the sensor to the system (r2) should be much larger than th e thermal resistance of the sens or to ambient (r1). for rapid response, the thermal mass connected to the sensor (c2) should be minimized. a practical example of this along with some specific numbers is discussed in "appendix e? thermal model for a sensor on a paddle" on page 38. 3.2.3. optimizing humidity response the humidity sensor element of the si7006, si7013, si7020, si 7021, and si7034 devices have a response time of 17?18 seconds at room temperature. for the si7007, si 7022, and si7023 devices, the humidity sensor response time is six seconds at room temperat ure. if the humidity sensing element is placed within a cavity, this will be affected by the following factors. ?? the temperature of the cavity compared to the air temperature ?? as discussed in "2.1.humidity sensor technology" on page 4, as air is heated or cooled, the air humidity will decrease or increase by 5% per degree of heating or co oling. the sensor and surrounding air must track the ambient temperature in order to accurately measure humidity. ?? the size of the cavity versus the size of the opening ?? generally, the opening should be comparable in size of the cavity in order to avoid slowing of the response. ?? porous materials within the cavity can absorb or em it humidity and dramatically affect the response. organic vapors, such as those from glue, can also produce a response. these effects are worse if the cavity opening is small compared to the cavity size. th e cavity should be made of materials, such as hard plastic, metal, or hard rubber. any sealants should be fully cured. 3.2.4. protection against liquids and dust like most other ic type humidity sensors, the si70xx humidity sensor s measure humidity by the change in dielectric constant of a porous po lyimide film. conductive dust particle s will cause errors in the readings, particularly if they become lodged in the humidity sens ing film. organic solvents can cause a semi-permanent shift in humidity readings. even liquid water can have a permanent effect on humidity response if it leaves a mineral residue after evaporation. generally, a porous cover is used to protect against dust and liquids. typically, the cover is made of expanded polytetrafluoroethylene (eptfe). polytetrafluoroeth ylene is often known under the name brand teflon ? , and eptfe is often known under the name brand gore-tex ? . the effectiveness of the cover is expres sed by the ingress protection or ip rating. a cover with a rating of ip67 is recommended, which means the cover is completely dustproof and can withstand water immersion to a depth of at least one meter. tambient sensor tambient system r3 r2 r1 c1 c2 AN607 8 rev. 1.7 most of the si70xx humidity and temperature devices are offered with a factory installed cover that is rated ip67. the cover is solder resistant (it can withstand a peak tem perature of 260 c) and has a pore size of 0.25 m, so it blocks all dust while passing water vapor. the cover will bl ock liquid water at pressure equivalent to ov er one meter depth. as a practical test of the effectiveness of the cover, si70 21 devices were subjected to a cigarette smoke test. in this test, si7021 devices were placed in a 3 liter jar with 20 lit ci garettes. during the time the cigarettes were burning, a small air gap was allowed so the cigarettes would not ex tinguish. after burning was complete, the air gap was removed, and the parts were allowed to sit in the smoke for 24 hours. use of the cover reduced the maximum shift from this extreme exposure from almost 40% rh to ab out 6% rh. in another test in which 100 cycles of condensation were allowed to form on the parts, the cover reduced the shift in reading from an average of 2.0% to 1.57%. the cover has a minimal effect on response time. 3.2.5. compensation for heat sources and optimizing transient response in some cases, it is not po ssible to completely isolate the sensor from the system for aesthetic reasons, and it is desirable to still obtain a fast an d accurate measurement of the am bient temperature and humidity by compensating for the effects of the system. in these cases, it is best to optimize the sensor loca tion as much as possible using the above guidelines. to compensate for the system effects, either the system mu st have a known effect, or an additional temperature sensor (typically a thermistor) is ne eded. the thermistor should be placed to measure the temperature of the system in the place where it is having the most in fluence on the humidity and temperature sensor. in these cases, the si7013 se nsor is a good choice because it has an auxiliary a/d for digi tizing the thermistor voltage and a linearization engine for converting this to temperature. 3.3. dealing with conde nsation and high humidity prolonged exposure to high humidity will cause gradual drift of th e humidity sensor readin gs. all members of the si70xx family have on-chip heaters that can be used to he at the chip to counter local high humidity and reduce this drift. condensation will also cause erroneous readings. if the co ndensation forms on the pol yimide film, there can be permanent shifts in sensor accuracy due to residue left after evaporation. the hydrophobic filter prevents liquid water from penetrating, so condensation on the outside of the part will generally not result in condensation on the polyimide sensing film of the si70xx. ho wever, if condensation forms on top of the filter, re adings will be high until it evaporates. also condensation on th e pcb can affect reliability and signal integrity. turning on the heater will reduce the chance of condensation form ing and will also evaporate condensation. however, turning on the heater will affect the local relative humidity (see also ?estimating rh with heating? on page 28. for example, turning on the heater with the control se tting 0x3 heats the sensor about 5 c (depending on pcb design and airflow), which results in a ~30% drop in local rh. however, due to variability in air flow and heater current, the si70xx heating can vary 2 c making rh readings with the heater on unreliable. depending on the nature of the application, there are several ways of dealing with this: ?? the amount of heating can be measured or characterized. for example, turn the heater on and off a few times, and use the on-chip temperature s ensor to measure the amount of heating. ?? the air ambient temperature can be sensed with a separate sensor, and rh can be calculated ?? while the rh reading is not accurate , the dew point reading is fairly accurate (although generally about 1 c low) with the heater on. if dew point is the only concern, it can be calculated from the humidity and temperature and then subtract 1 c. the si7006, si7013, si7020, si7021, and si7034 have opti ons for increasing the heater current up to 94 ma with vdd = 3.3 v. depending on the pcb layout and thermal design, it is possible to get junction temperatures well in excess of 100 c. shifts in sensor readings from previous exposure to high humidity can be reversed by turning on the heater with a sufficiently high setting to get the chip temperature over 100 c for approximately 24 hours. AN607 rev. 1.7 9 3.4. pcb layout the si70xx should be thermally isolated from the equipmen t connected to it to prevent heat from the equipment from affecting rh. the si70xx should be thermally imme rsed in the ambient environment it is intended to sense. one strategy for accomplishing this is to put the si70xx on a small pcb and run a ribbon cable to the host processor. the small pcb should be placed away from heat sources and should be placed in the ambient environment as much as possible. that said, even with the hydrophobic filter, keeping dust, liquids and cleaning agents away from the sensor is required. 3.5. design and br ing-up checklist ?? be sure the sensor is placed away from heat sources and exposed to the environment being measured. ?? prevent the active area of the sensor from being exposed to liquids, dust, and other contaminants as well as sunlight or other uv sources. the optional filter cover available with si70xx parts serves this purpose and is compatible with soldering. ?? generally, avoid the use of ground planes around the si70xx, which co uld conduct heat from external sources. route the ground connection. ?? do not connect unused pins. make sure the csb pin of the si7015 is low prior to starting i 2 c communications. ?? the si7005 should not be on the same bus as other i 2 c devices when it is active. it acknowledges data bytes that match its address. this issue has been resolved with other members of the si70xx family ?? be sure to meet all of the timing and level requirem ents of the device. the si7005 can tolerate sda or scl higher than v dd and has 8.5 ma drivers. the si7006, si7013, si7020, and si7021 have 2.5 ma drivers and do not tolerate i 2 c pins higher than v dd . the si7034 is a 1.8 v part but can tolerate 3.3 v on its inputs. it's drive strength is 1.5 ma. see also application note, ?an883: low-cost i 2 c level translator? for a low cost i 2 c level translator circuit. ?? route the i 2 c signals away from analog nodes and noisy digital nodes. ?? use 0.1 f bypass capacitors on v dd placed close to the sensor. ?? pay careful attention to i 2 c protocols, such as start and stop c onditions, the repeated start of a read transaction, and proper treatment of the acknowledge bit. ?? allow adequate time for initialization (per data sheet). ?? if the optional thermistor sensing of si7013 is used, make sure the thermistor is thermally isolated. if there are long leads to the thermistor, use a twisted pair. av oid noise pick up; use either a shield or capacitive filter. 3.6. si70xx self test the following steps define a reliable test of the si7006, si 7013, si7020, si7021, and si 7034 family that uses the integrated heater: 1. read and write all i 2 c registers checking for expected val ues and capability of modifying where appropriate 2. perform an rh and temperature measurement. 3. turn on the heater and wait 60 seconds. 4. check for delta temperature with heater on. this can be adjusted changing the heater setting. a setting of 0x3 will give over 3 c. 5. check for delta rh is > rhinitia l 4x (delta temperature in c). AN607 10 rev. 1.7 3.7. esd considerations it is desirable to expose the si70xx sensor to the enviro nment. for the sensor to respond to the environment there must be a way for the air being sensed to reach the sensor (the environmental access port). this means that the sensor may also be exposed to esd as specified in iec 61000-4-2 with esd peak voltage of up to 15 kv. for the humidity sensors, when the cover is not used and t he esd source is directly over the package opening, it is possible for esd to arc into the sensor area and cause damage. this ca n be avoided by using of the silicon labs filter cover or by placing the si70xx so that the sensor opening is offset from the environmental access port. the above approaches will prevent esd discharge into th e sensor area, but esd discharge to the leads may still be possible. the best practice for esd protection of the leads is to arrange the sensor placement and environmental access so that high-level esd events will preferentia lly be directed to groun d (i.e., have an exposed ground trace or ground shield closer to the environmental access port than the sensor). if grounded, a metal case is used; this is also effective for esd protection. if it is not possible to protect the leads of the device from esd, unused leads should be connected to vdd. high- quality esd protection diodes can be used on leads that have signals on them. the esd protection device should be rated for more than 15 kv immunity and should limit esd voltage peaks to less than 10 v. some examples that have been tested include vishay msp3v3 and comchip cpdq3v3u-hf. AN607 rev. 1.7 11 4. humidity and temperature sens or special handling considerations 4.1. product storage the si70xx are shipped in sealed anti-static bags. the sensors may be stored in a humidity and temperature controlled (rh: 20% to 60%, temp 10 c to 35 c) environment for up to one year after being removed from the bag prior to assembly (the moisture se nsitivity rating is msl2). do not stor e the humidity sensors in polyethylene bags (typically blue, yellow, or pink) because these emit gases that can affect the sensor. metallic, anti-static, sealable, moisture-barrier bags are recommended for storag e. do not use sealants or tapes to seal inside the packaging. 4.2. use of conformal coat ing and under-fi ll materials use of conformal coating or un der-fill material is possible with the following cautions: ?? conformal coatings and under-fill material must not be allowed to come directly in contact with the humidity sensing layer of the si70xx as they will adhere to the polyimide film and cause a permanent shift in the humidity sensor readings. even very small particles can have a significant effect. ?? generally, materials that outgas or give off an odor have the potential to affect sensor performance. following are general recommendations to avoid humidity sensor drift from fumes: ?? cover the si70xx during the application process with a co ver that forms a seal on t he device such as kapton? kppd-1/8 polyimide tape. ?? the optional silicon laboratories protective cover will not bl ock fumes, but it will block liquids and particles. it may be effective if the fume concentration is not high. ?? use low volatile organic compound (voc) materials. ?? immediately cure the material in a well-ventilated environment. we recommend that a test run be performed to measure humidity in a controlled chamber before and after the coating process to make sure no shifts are occurring. if a humidity-controlled test chambe r is not available, perform a side-by-side test with one board coated and one board not coated. these cautions are consistent with all polyimide-based relative humid ity sensors. in general, if a process has been qualified with a similar part from a different vendor, it will be acceptable for the si70xx devices. 4.3. assembly flow limit soldering iron rework to five seconds per lead, for compete rework use a new sensor as manual removal and soldering can shift senor accuracy outside of data sheet limits. avoid the use of hot air rework tools. figure 8. limit solder rework to five seconds or less the humidity sensor opening should generally be covered during soldering to prevent flux from getting on the sensor surface. further it is recommended that the si70xx only be soldered with standard reflow (no hand soldering or hot air tools). the hydrophob ic filter is compatible with standard reflow soldering. if the hydrophobic filter cover is not used, kapton tape will serve the same purpose, although it has to be removed after soldering. soldering iron touch-up is possible if liquid flux is not needed and care is taken to avoid excessive heating (five seconds per lead). if complete rework is needed, the re commended method is to use a new part and reflow the entire board. ? ? ? ? ? ? ? 5 ? seconds ? maximum AN607 12 rev. 1.7 the si70xx filter cover is compatible with standard reflow soldering. it provides lifetime protection against dust and liquids and should not be removed after soldering. figure 9. do not remove si70xx protective cover the reflow should follow jedec standards for lead free solder and reflow with a peak temperature of less than 260 oc. ?no clean? solder paste should be used. the us e of an ultrasonic bath with alcohol for cleaning after soldering is specifically not recommend ed. the si70xx sensor opening must be kept clean and free of particulates during assembly; the pre-installed white filter cover will protect the openi ng from particulates . ensure the sensor opening does not come into contact with conformal coatings. do not expose the si70xx to volatile organic compounds or solvents. if installed, do not re move the white filter cover from the devices. the use of water-soluble flux and water rinse after solderin g is permissible if done with care. the use of di water is recommended. if the hy drophobic cover is used, a spra y pressure of less than 40 psi will prevent water entry into the cover. without the cover, care would need to be taken to avoid particles from the water or from blow drying to contaminate the sensor area. the high-temperature solder ing process will introduc e a recoverable shift in the sensor indication. generally, accuracy will be back within tolerance limits within 48 hours of soldering if the sensor is stored in normal ambient conditions with ~50%rh. 4.4. sensor sensitivit y to chemicals and vapors the si70xx is sensitive to many chem icals and fumes. notably, household cl eaning agents, such as ammonia, are known to cause sensor readings to drift. to maintain accuracy of the si70xx, avoi d exposure to chemical fumes and contaminants. ?? inert dust (e.g., talc ) is essentially benign. ?? excessive amounts of dust can slow response. ?? contaminants or particles embedded in the polyimide can affect the rh accuracy. ?? certain polyethylene bags will ou tgas and damage the sensor. ?? bleach, hydrogen peroxide, ammonia, and other chemicals can affect or damage the sensor. 4.5. recovering calibration after hi gh humidity or chemical exposure typically, initial accuracy can be recovered by baking th e sensor at 125 c for 12 hours followed by ~2 days storage period in normal ambient c onditions with ~50%rh. high rh exposur e (i.e., 75% rh for 12 hours) will accelerate the post-bake recovery, but, after high rh exposure, approximately two days at normal rh is still recommended for the device to fully recover its accuracy. ???????????????????????????????????????? ? do ? not ? remove ? protective ? filter AN607 rev. 1.7 13 4.6. relative humidi ty sensor accuracy to determine the accuracy of a relative humidity sensor, it is placed in a temperat ure and humidity controlled chamber. the temperature is set to a convenient fixed valu e (typically 25?30 c) and the relative humidity is swept from 20 to 80% and back to 20% in the following steps: 20% ? 40% ? 60% ? 80% ? 80% ? 60% ? 40% ? 20%. at each set-point, the cham ber is allowed to settle for a period of 30 minutes before a reading is taken from the sensor. prior to the sweep, the devic e is allowed to stabilize to 50%rh. th e solid trace in figure 10, ?measuring sensor accuracy including hysteresis,? shows the result of a typical sweep. figure 10. measuring sensor accuracy including hysteresis the rh accuracy is defined as the dotted line shown in fi gure 10, which is the average of the two data points at each relative humidity set-point. in this case, the sensor shows an accu racy of 0.25%rh. the si70xx accuracy specification includes: ?? unit-to-unit and lot-to-lot variation ?? accuracy of factory calibration ?? margin for shifts that can occur during solder reflow the accuracy specification does not include: ?? hysteresis (typically 1%) ?? effects from long term exposure to very humid conditions ?? contamination of the sensor by particulates, chemicals, etc. ?? other aging related shifts ("long-term stability") ?? variations due to temper ature. rh readings will typically va ry with temperatur e by less than ? 0.05% ? c . AN607 14 rev. 1.7 4.7. hysteresis the moisture absorbent film (polymeric dielectric) of the humidity sensor will carry a memory of its exposure history, particularly its recent or extreme exposure history. a sensor exposed to relative ly low humidity will carry a negative offset relative to the fact ory calibration, and a sensor exposed to relatively high humidity will carry a positive offset relative to th e factory calibration. this fact or causes a hysteresis effect illustrated by the solid trace in figure 10. the hysteresis value is the difference in %rh between the maximum absolute error on the decreasing humidity ramp and the maximum absolute error on the increasing humidity ramp at a single relative humidity setpoint and is expressed as a bipolar quantity relative to the average error (dashed trace). in the example of figure 10, the measurement uncertaint y due to the hysteresis effect is 1.0%rh. 4.8. prolonged expos ure to high humidity prolonged exposure to high humidity will result in a gra dual upward drift of the rh reading. the shift in sensor reading resultin g from this drift will generally disappear slowly unde r normal ambient cond itions. the amount of shift is proportional to the magnitude of relative humidi ty and the length of exposure. in the case of lengthy exposure to high humidity, some of the resulting shift may persist indefinitely under typical conditions. it is generally possible to substantially reverse this effect by baking the device as described in the following section. 4.9. bake/hydrate procedure after exposure to extremes of temperature and/or humi dity for prolonged periods, the polymer sensor film can become either very dry or very wet, in each case the result is either high or low relative humidity readings. under normal operating conditions, the induced error will diminish over time. from a very dry condition, such as after shipment and soldering, the error will diminish over a few days at ty pical controlled ambient conditions, e.g., 48 hours of 45 %rh 55. however, from a very wet condition, recovery may take signi ficantly longer. to accelerate recovery from a wet condition, a bak e and hydrate cycle can be implemente d. this operation consists of the following steps: ?? baking the sensor at 125 c for 12 hours ?? hydration at 30 c in 75% rh for 10 hours following this cycle, the sensor will return to normal operation in typical ambient conditions after 48 hours. AN607 rev. 1.7 15 5. an introduction to humidity atmospheric air normally contains water vapor and can be thought of as a mixture of ideal gasses. dry air (no moisture content) is the combination of approximately (on a mole basis) 78.09% n2, 20.95% o2, 0.93% ar and 0.03% co2 and trace elements. the amount of water vapor found in air depends on available liquid water (or ice), temperature, pressure and ranges from nearly zero to the point of saturation called ?dew point (frost point)?. water vapor enters the air by evaporation due to the vapor pressure of water or ice. 5.1. vapor pressure there are a few key concepts to keep in mind when disc ussing vapor pressure. pure water vapor pressure, p, is due to water vapor pressure over water or ice without the pr esence of other gases such as air. in combination with air, the actual water vapor pressure is increased by a dimensionless factor referred to as a water vapor enhancement factor, f. this factor is a weak function of temperature and pressure and is approximately 0.47% at sea level and 20 c. the actual vapor pressure of water vapor, p', is the pure water vapor pressure, p, multiplied by the enhancement factor, f. dalton's law states that the total pressu re of a mixture of gasses is equal to the sum of the partial pressures of each component gas and assumes the combination of gasses behaves like an ideal gas. in an ideal gas mixture such as air, the total pressure is the sum of the partial pressures of each gas. the pressure of air at any point can be calculated as follows. note that p h2o is the actual vapor pressure of water in air referred to as p'. there are several useful equations when working with vapor pressure. the best one to use depends on the available information you have about the problem you need to solve, the range of conditions for the problem the degree of accuracy required and the computational re sources available. these equations and their range of application are discussed in detail in appendix b. pressures referred to in this document are absolute (not gauge) unless otherwise specified. 5.2. temperature the relative humidity value can change significantly with even slight variations in temperature. for example, a 1 c change in temperature at 35 c and 75% relative humidi ty will introduce a ?4% rh ch ange. a higher temperature increases the ability of air to absorb moisture and a lower temperature decreases the ability of air to absorb moisture. temperature changes can introduce moisture vari ations in an air mass if condensation occurs or through secondary impacts such as changing the moisture absorption or desorption of environmental materials in an enclosure. for humidity sensors that re spond in proportion to relative humidity and not absolute humidity, the issue of temperature measurement error is not significant unles s conversion to dew point, absolute humidity or any other measurement of water vapor in the air is required. in th e case of a dew point calculation, a 1 c error in the measurement of the temperat ure will produce approximately a 1 c error in the calculation of the dew point. this temperature dependency not only emphasizes the import ance of accurate temperature measurement, it also highlights the necessity of thermal stability, which can be di fficult to achieve. even if the temperature and humidity measurements are taken in relatively close proximity, there can be considerable differences in corresponding levels of humidity and temperature. to achieve the most accurate measurement it is best if the humidity and temperature measurements are taken as close as possible to each other?ideally co-located on the same chip. p ? pf p 1.0047 ?? == pp n2 p o2 p h2o p ar p co2 ++ ++ = AN607 16 rev. 1.7 5.3. evaporation the average energ y of molecules in a liquid is directly proportiona l to temperature. the vapor pressure of water will increase as it is heated. when it reaches 100 c at sea level, the va por pressure will equa l the atmospheric pressure (approximately 760 torr or 101.325 kpa) and the water will bo il. the energy (vapor pressure) of water molecules in an open partially filled c ontainer of liquid water (or ice), figure 11 , is such that some of the molecules have enough energy to escape the attractive forces hold ing the water together and evaporate into the atmosphere. eventually all of the water will evaporate assuming the air is not saturated. if the container is sealed, figure 12, water molecules will evaporate into the air space above th e water and some of the ev aporated water molecules will condense back into the liquid water. the rate of evaporation will exceed the rate of condensation until the air above the water becomes saturated with water vapor. once the air is saturated the rates of evaporation and condensation will be equal (the vapor pressure will equal the saturation pr essure inside the closed c ontainer). evapo ration takes place at the surface of the liquid while boiling can take place throughout the volume of the liquid. figure 11. water in open container evaporation > condensation humidity in the open air is increasing AN607 rev. 1.7 17 figure 12. water in closed container figure 13. water boiling boiling starts at the liquid surface when the vapor pre ssure equals the atmospheric pressure. as temperature increases, bubbles form below the surface as the vapor pressure increases to equal the atmospheric pressure plus the additional pressure due to the weight of the colu mn of liquid above the bubble. as temperature and vapor pressure continue to increase, bubbles will form thro ughout the entire vo lume of liquid as shown in figure 13. water vapor is completely ab sorbed into the air which can cause the air in close prox imity to the boiling water to become locally saturated. visible st eam consists of very small water dr oplets which have condensed out of this localized super-saturated air. evaporation = condensation saturated vapor p?>p evaporation >>> condensation humidity in the open air is increasing steam AN607 18 rev. 1.7 6. how humidity is quantified humidity represents the amount of water vapor contained in the air and can be quantified in many ways. several of the terms describing humidity are defined differently fo r meteorology applications and thermodynamics or chemical engineering applications. for this reason, it is important to understand the context of the application. the following is a brief description of the most common terms for quantifying humidity. 6.1. absolute humidity absolute humidity in the context of meteorological applications, sometimes referred to as ?volumetric humidity?, is defined as the mass of water vapor dissolved in a total volume of moist air at a given temperature and pressure. typical units are g/m3 or kg/m3. the value of absolute hu midity defined in this manner changes with temperature and pressure and is inconvenient to use in many engineering applications. absolute humidity for use in thermodynamics or chemical engineering applications is defined as the ratio of water vapor mass to dry air mass. typical units are kg/kg. other names for this ratio include mass mixing ratio, humidity ratio, mass fraction or mixing ratio. this quantity is simpler to use and more accurate for mass balance or heat calculations. due to the conflicting definitions of absolu te humidity, caution is required when using this term. 6.2. specific humidity specific humidity, yw, can be defined for meteorology a pplications as the ratio of water vapor mass per mass of moist air expressed as g/kg or kg/kg. specific humidity is constant with changes in temperature and pressure for conditions above the dew point and is a useful quantity in meteorology. the rate of evaporation of water is directly proportional to specific humidity. 6.3. relative humidity relative humidity is the ratio of actual water vapor pres ent in air with the amount of water vapor that would be present in air at saturation, expressed as a percent age. the official symbol for relative humidity is ? although rh, %rh, rh, or %rh are commonly used. relative humidity can be expressed as the ratio of the actual vapor pressure, p', to the saturation vapor pressure, p s ?, at a constant temperature over a plane of liquid water. 6.4. dew point upon heating, the capacity of air to absorb moisture increases. consequently, the relative humidity of air decreases as the air is heated. conversely, as moist air is cooled, its capacity to absorb moisture decreases and relative humidity increases. the dew point is the temperature, assuming constant pressure, moist air is saturated (reaches 100% relative humidity) and cannot absorb additi onal water. as the temperature is decreased past the dew point, moisture condenses until the air is saturated (reaches 100% relative humidity) at the new lower temperature. 6.5. frost point frost point is the same as dew point over solid ice wh ere the condensate is frost instead of liquid water. rh (%) p ' p s' ?? ? 100 ? = AN607 rev. 1.7 19 7. humidity measurement for relative humidity measurements, it is not necessary to measure the ambient temperature unless you are using a psychrometer. however, to determine the dew point or ab solute humidity, the ambient air temperature is required. accurate air temperature measurements can be a significan t challenge, since air is a poor thermal conductor and the temperature at any given point can be impacted by air currents and temperature gradients. it is very important to understand the dynamics of your measurement system and the dynamics of the environment being measured. before taking a measurement, you always need to wait long enough to ensure the temperature and humidity are stable and the sensor(s) of the measurement instru ment are in equilibrium with the ambient conditions to be measured. a wide range of techniques are employed to measure humid ity. these range from simple mechanical indicators, to highly complex and expensive analytic al instruments. in general, measuring humidity (dew point, absolute humidity, specific humidity, mixing ratio, relative humidity or equi valent wet bulb temperatur e) is not a trivial task. many of the instruments currently available have poor accuracy, narrow-bandwidth, contamination issues, hysteresis and measurement drift over temperature and ti me. regular calibration is re quired on some instruments, which is both inconvenient and expensive. some instru ments are large, awkward, and expensive. discussed below, are different methods of humi dity sensing. the new generation of humidity sensor technology used in silicon labs? solid state humidity sensor s provide superior accuracy, minimum dr ift, low cost, low power, small size and ease of use. 7.1. psychrometer a psychrometer is the oldest method for measuring humidity more commonly known as the wet bulb/dry bulb method. a psychrometer consists of two thermometers, one with an ordinary dry bulb and the other with a moist cloth covering the bulb (wet bulb). evaporation from the moist cloth lowers the wet bulb thermometer's temperature. the wet-bulb thermometer shows a lower temper ature, twb, than the dry-bulb thermometer, tdb, if the air is not saturated with water vapor. the temperatures are the same in saturated air. the amount of evaporation is dependent on the relative humidity of the air (more evaporation and lower twb with lower rh). given the dry bulb and wet bulb temperatures the relative humidity is looked up on a psychrometric chart. for example, at sea level if tdb = 25 c and twb = 18 c, rh = ~50%. looking up the %rh on a chart for every measurement is both time-consuming and cumbersome but can be automated with a microcontroller. a psychrometric sensor can ac hieve good precision with %rh resolution s of 0.01% for humidity ranges from 10?100% at temperatures from 0 to 60 c, and accuracy of 1%. the disadvantages of a psychrometric sensor are a slow response time, large physical size, the need to ke ep one thermometer bulb wet, the need to have airflow around the wet bulb, and high cost. 7.2. chilled mi rror hygrometers the chilled mirror hygrometer is consid ered the most accu rate and reliable hygrometer . chilled mirror hygrometers use a cooled mirror with an optoelectronic mechanism to detect condensation on the mirror surface at an accurately measured temperature. the system is confi gured to reflect led light off a mirror at an angle of approximately 45 degrees with a photo- transistor detecting the reflected lig ht. the temperature of the mirror is electronically controlled, typically with a peltier-effect device. the system works by cooling the mirror's surface below ambient temperature until condensation forms, causing the led's light to scatter resulting in a sudden drop in the output of the photo-tr ansistor. the surface temperature of the mirr or is read using an accurate temperature sensor such as a thermistor. the temperature at which condensation forms is the dew point. all humidity values can be calculated from the dew point. the mirror temperature can be controlled with a feedback loop to continuously track the dew po int. the chilled mirror is the most stable an d accurate method to determine relative humidity. however, it is crucial to keep the mirror cl ean, provide a method of clea ring the condensation and to ensure that the temperature sensor and mirror are of high quality. this method operates over the full humidity range (0-100%rh) and can be used for numerous gases at many pressures. ch illed mirror hygrometer instruments are bulky and very expensive. AN607 20 rev. 1.7 7.3. mechanical hygrometers mechanical hygrometers use sensing elements relying on a mechanical property of the sensor varying with humidity. the most common example is the animal hair hygrometer, which uses a piece of animal hair kept under tension. as humidity increases the hair becomes more flexible and stretches. a strain gage monitors the displacement caused by the hair stretching with a change in the moisture content of the air. the output of the strain gage is directly proportional to the relative humidity and is usually indicated on a mechanical meter. mechanical hygrometers are generally compact, light weight, reliable, and inexpensive. accuracies, however are in the 10% range. 7.4. electronic humidity sensors electronic humidity sensors typically use either a change in resistance or capacitance to measure humidity. these sensors have become a popular choice because technology advances have made them accurate, compact, stable and low power. a capacitive sensor cons ists of two electrodes, separated by a dielectric. as the water vapor in air increases or decreases, the sensor's dielectric constant changes producing a higher (or lower) capacitance measurement corresponding to the humidity level. a resist ive sensor consists of two electrodes, separated by a conductive layer. as the humidity in the air increases (or decrea ses) the conductivity of the sensing layer changes, altering the resistance between the tw o electrodes. new techniques for producing thin films have made these types of sensors, accurate, stable, and easy to manufacture in large quantities. the choice of material assures fast response times with little hysteresis. th e accuracy of electronic sensors is limited by their drift over time caused by wide variations in temperature and hu midity or the presence of pollutants. the si70xx humidity senors use a mim capacitor as the re ference and perform a high-accuracy 24-bit conversion using a sigma delta conversion approach. each part is fa ctory calibrated for capacitance to rh with a minimum offset and slope correction. later me mbers of the family include non-linearity correction and temperature compensation. silicon laboratories humidity sensors use the capacitance change due to moistu re absorption of a polyimide film to sense humidity. the polyimide film is deposited over a metal finger capaci tor and exposed to the ambient via an opening in the package. the polyimide ma terial and sensing capacitor has been selected for excellent stability. an optional expanded polytetrafluoroethylene (eptfe) hydrophobic filter provides protection against dust and most liquids. the polyimide film is thin (<5 m) and very responsive to humidity (response time of less than 10 seconds). the hydrophobic filter has little impact on the response time. while the si70xx sensors are largely conventional, mixe d-signal cmos integrated circuits, relative humidity sensors in general, and those based on capacitive sensin g using polymeric dielectrics in particular, have unique application and use requirements that are not common to conventional (non-sensor) ics. chief among those are: ?? the need to protect the sensor during board assembly, i.e., solder reflow, and the need to subsequently ?? rehydrate the sensor. ?? the need to protect the senor from damage or contamination during the product life-cycle. ?? the impact of prolonged exposure to extremes of temp erature and/or humidity and their potential effect on ?? sensor accuracy. ?? the effects of humidity sensor ?memory?. each of these items is discussed in more detail in the following sections. AN607 rev. 1.7 21 8. humidity control for thermal comfort 8.1. general considerations humans are sensitive to humid air, because the human body uses evaporative cooling as the primary mechanism to regulate temperature. when the relative humidity and dew point are high, the rate of perspiration evaporation from the skin decreases because the amount of water vapo r in the air is already close to saturation. because humans perceive the rate of heat transfer from the body, rather than temperature itself, we feel warmer when the relative humidity is high. relative humidity is a useful i ndication of how hot the weather ?feels? and is more intuitive and easier to measure than other quanti fications of water vapor in the air. air conditioners are designed to maintain between 40?60% relative humidity in the occupied space. humans tend to react with discomfort to dew points >61 f (16 c). the body perspires and produces sweat to cool down. high relative humidity an d consequently a high dew point preven t the evaporation of sweat and reduce evaporative cooling. the body may overheat, resulting in discomfort. lower dew points, <50 f (10 c), generally correlate with lower ambient temperatures requiring the body to be less dependent on evaporative cooling. a lower dew point can be achieved with a high temperature at extremely low relative humidity allowi ng for effective cooling. table 1. human reaction to humidity* dew ? point ? temp ? ( f) rh ? at ? 90 fh u m a n ? perception > 75 > 62% extremely uncomfortable 70 to 74 52% to 60% quite uncomfortable 65 to 69 44% to 52% somewhat uncomfortable 60 to 64 37% to 46% comfortable but humid 55 to 59 31% to 41% comfortable 50 to 54 31% to 37% very comfortable < 49 < 30% a bit dry *note: see lawrence, mark g., ?the relationship between relative humidity and the dewpoint temperature in moist air?a simple conversion and applications?, american meteorological society february 2005. AN607 22 rev. 1.7 8.2. heat index the heat index (hi), also called ?apparent temperature? co mbines air temperature and relative humidity to estimate how ?hot? a human will perceive the ambient condition s to be. the heat index is calculated for ambient temperatures above 27 c (81 f) and dew points above 12 c (54 f) (relative humidity above 40%). heat index is calculated with the following equation 2 . 1. www.shorstmeyer.com/wxfaqs /humidity/humidity.html 2. www.srh.noaa.gov/images/ epz/wxcalc/heatindex.pdf 3. www.crh.noaa.gov/pub/heat.php 8.3. wind chill wind chill, also called wind chill temper ature, wind chill factor or wind chill index, expresses the perceived air temperature on skin exposed to wind. wind chill is defined for temperatures at or belo w 10 c (50 f) and wind speeds greater than 4.8 kilometers/hour (3.0 mph). wind chill is calculated by the following equations 4 . 4. http://web.uvic.ca/~e os340/wind_chill.pdf table 2. human reaction to heat index, hi, in shady light wind conditions* heat index, hi, f human impact 80 to 90 fatigue possible with prolonged exposure and physical activity 90 to 105 sunstroke, heat cramps and heat exhaus- tion are possible 105 to 130 sunstroke, heat cramps and heat exhaus- tion likely. heat stroke possible > 130 heat stroke is highly likely with continued exposure *note: see www.crh.noaa.gov/pub/heat.php. hi 42.379 ? 2.04901523t ?? 10.14333127rh ?? 0.22475541trh ?? ? 6.83783 10 3 ? t 2 ? ?? ? 5.481717 10 2 ? rh 2 ? ?? ? 1.22874 10 3 ? t 2 rh ? ?? 8.5282 10 4 ? trh 2 ? ?? 1.99 10 6 ? t 2 rh 2 ? ?? where: hi = head index in degress farenheit t = ambient temperature in degrees farenheit rh = relative humidity in % ? ++ ++ = t wc 35.74 0.6215t a 35.75v 0.16 ? 0.4275t a v 0.16 where: t wc wind chill temperature in degrees farenheit t a ambient temperature in degrees farenheit v wind velocity in miles-per-hour = = = ++ = AN607 rev. 1.7 23 8.4. thermal stress heat index and wind chill are a measure of how the temperature is perceive d by humans and can be referred to collectively as ?apparent temperature? or ?relative outdoor temperature?. the human body looses heat by conduction and convection. the rate of heat loss by the body depends on the amount of exposed skin and the wind speed. the human body responds to heat loss by attempting to maintain its surface temperature. rapid heat loss results in both the perception of lower temperatures and an actual greater heat loss as the body attempts to maintain body temper ature on exposed skin increasing the risk of hypothermia, frostbite and death. 8.5. environmental quality traditionally an environment was controlled based on a temperature measurement. in recent years the relative humidity measurement has become just as important. humidity control is especially important in living, storage, and manufacturing sites. control of temperature and relative humidity is also critical in the preservation of many materials including medication, food, fabric and wood products. unacceptable levels of temperature and/or relative humidity contribute significantly to the breakdown of ma terials. heat accelerates deterioration, whereas high relative humidity provides the moisture necessary, to promote harmful chemical reactions and, in combination with high temperature, encourages mold growth and insect acti vity. extremely low relative humidity, which can occur in winter in centrally heated buildings, may lead to desicca tion of some materials causing them to become brittle. large fluctuations in temperature and relative humidity are damaging due to expansion and contraction which can accelerate deterioration. installation and operation of adequate climate controls to meet preservation standards will retard the deterioration of materials considerably, while maintaining a comfortable environment for humans. for example, luthiers recommend keeping fine wooden musical st ringed instruments such as violins, cellos and guitars at a rh between 45% and 55% and temperatures between 16 c (55 f) to 21 c (70f) to prevent warping and splits in the wood. the ideal temperature and re lative humidity will vary dependin g on the material and application. typically, a steady temperature of 16 to 21c and a re lative humidity between 30% and 60% suffices for many applications. low power battery operated temperature and humidity sensors can be used to monitor conditions during shipment or storage for food and a variety of other materials. table 3. wind chill temperature chart* v\t a 40 f 30 f 20 f 10 f 0 f ?10 f ?20 f ?30 f 10 mph 34 21 9 ?4 ?16 ?28 ?41 ?53 20mph30174?9 ?22 ?35 ?48 ?61 30 mph 28 15 1 ?12 ?26 ?39 ?53 ?67 40mph2713?1?15 ?29 ?43 ?57 ?71 50mph2612?3?17 ?31 ?45 ?60 ?74 *note: the shaded area indicates a danger of frostbite. AN607 24 rev. 1.7 a ppendix a?i ndustry s pecifications and g uidelines ?? a.1 ansi/ashrae standard 55 ?? a.2 bs1339 ?? bs 1339-1:2002 part 1: terms, definitions and formulae ?? bs 1339-2:2009 part 2: humidity calculation and tables - user guide ?? bs 1339-3:2004 part 3: guide to the measurement of humidity ?? u.k. national physics laboratory guide to rh measurement ?? http://www.npl.co.uk/publications/go od-practice-online-modules/humidity/ the following topics can be found on wikipedia: ?? relative humidity ?? dew point ?? i 2 c ?? ip rating ?? polymers ?? wave soldering si70xx certificate of compliance (with web link) AN607 rev. 1.7 25 a ppendix b?e quations for v apor p ressure and h umidity c alculations many equations have been developed to express humidity parameter relation ships. they can generally be broken down into two groups, those developed from thermodyna mic principles and equations empirically derived from experimental data. the clapeyron and clausius-clapeyron equations will be pr esented as examples of the first group and the sonntag, magnus and antoine equations as examples of empirically derived expressions. the empirically derived expressions are generally easier to us e but have limitations to the range of use and accuracy. approximations may be useful to further simplify calc ulations and conversions between humidity parameters as long as the limitations imposed by the approximations are fully comprehended. the use of these equations and approximations will be discussed below. clapeyron equation the clapeyron equation is based on on e of the maxwell thermodynamic relationships contains no approximations and provides an exact solution. it considers saturati on pressure and temperature, the change of entropy associated with a change of phase and the change in volume of the two phases and represents the slope of the vapor-pressure curve. the clapeyron equation can be expressed as follows: the clapeyron equation is valid for all phase transitions (s olid/liquid, solid/gas and liquid/gas) and represents the slope of the phase change boundaries. the parameters in this equation that can be directly measured are temperature, pressure and volume. entropy and enthalpy can only be measured indirectly in terms of the other parameters. ? p ? ? t ------- - ? s ? v ------ - where: p ? saturation vapor pressure t temperature in k ? s entropy change between the two phases ? v volume change since: ? s ? h t ------ - across a phase transition = where: ? h enthalpy change between the two phases the clapeyron equation can be rewritten as follows: ? p ? ? t ------- - ? h ? vt ---------- -= = = = = = = AN607 26 rev. 1.7 clausius-clapeyron equation the clausius-clapeyron equation modifi es the clapeyron equation with two simplifying approximations that make this equation useful for ice to water vapor and liquid wate r to water vapor transitions. the first assumption is that the change in volume from liquid water to gas (water vapo r) or solid (ice) to gas (water vapor) is approximately equal to the volume of the gas (water vapor). the second approximation is the gas (water vapor) can be treated as an ideal gas. incorporating these assumptions in the clapeyron equation yields the following. rearranging: integrating as an indefinite integral and assuming ? h is constant: where: ? h = enthalpy change for phase change which varies between 2.501 x 106 and 2.257 x 106 j/kg in the range of 0?100 c r? = universal gas constant r = specific gas constant for wa ter which is 461.5 j/(k ? kg) rearranging: accuracy is best around the tem perature used to calculate c 2 . for example c2 = 2.53 x 10 11 pa at 0 c ? vv gas v liquid ? v gas ? vv gas v solid ? v gas ? = ? = v gas nr ? t p ------------- = ? p ? ? t -------- ?? ?? ? h t nr ? t p ? ------------- ----------------- - ? h rt 2 p ? ---------- - ---------- - == dp? p ? -------- ?? ?? ? h rt 2 ---------- - dt = p ? d p ? -------- ? ? h r ------ - 1 t 2 ------ ? dt p ? ?? ln c 0 + ? h ? r ---------- 1 t --- ?? ?? c 1 + = = p ? c 2 e ? h ? r ---------- 1 t --- ?? ?? = AN607 rev. 1.7 27 humidity-related calculations while the above expressions are physically based, they are difficult to solve and manipulate. hence, many approximate formulas have been developed. the two most common are the antoine equation and the magnus equation. many of these formulas contain coefficients that can vary depending on their source, its age and, in some cases, the context of the equations use. antoine equation this equation calculates saturation va por pressure. additionally; coefficients are available for this equation for a wide variety of vapors other than water. the coefficients used in the following equati on are for an air-water system and are optimized for use over the temperature range 0 to 100 c. magnus equation this equation calculates saturation vapor pressure as does the antoine equation. it has the advantage that it can be easily manipulated to find the dew point (t dp ) or frost point. where: p? s is pressure in pascals nm ?2 t is temperature in c over the range of ?40 to +50 c, t he best fit constants are as follows: for air with a vapor pressure p?, the dew point is defined as the temperature at which the water vapor would be saturated. thus, utilizing the relationship rh = 100 x p?/p? s a 1 b 1 c 1 reference 17.625 243.04 610.94 alduchov, oleg a., robert e. eskridge, 1996: improved magnus form approximation of saturation vapor pressure. j. appl. meteor. , 35 , 601?609 p s 23.19 3830 t 44.83 ? ------------------------ ? ?? ?? exp = t dp 3830 23.19 ps ?? ln ? -------------------------------------- 44.83 + = where: pressure in pascals nm 2 ? ?? temperature in k (c + 273.15) p ? sc 1 e a 1 t b 1 t + ----------------- ?? ?? ? c 1 ------- ?? ?? a1 p ? c 1 ------- ?? ?? ln ? -------------------------------- ? AN607 28 rev. 1.7 equation comparison the following graph compares the above equations: mark lawrence rule of thumb* this easy to use equation provides an estimate of change in dew point for a change in relative humidity or can be easily reversed to estimate a change in relative humidity for a known change in dew point. this approximation is valid for rh>50%. this rule of thumb says that the de w point temperature decreases approximately 1c for every 5% decrease in rh starting at tdp = t and rh=100%. this relationship is very handy if very little computational capability is available and the accuracy limitat ions and range of applicability are acceptable. *note: see lawrence, mark g., ?the relationship between relative humidity and the dewpoint temperature in moist air - a simple conversion and applications?, american meteorological society february 2005. td b 1 in rh 100 --------- - ?? ?? a 1 t b 1 t + ---------------- + a 1 in rh 100 --------- - ?? ?? ? a 1 t b 1 t + ---------------- ? --------------------------------------------------------- = AN607 rev. 1.7 29 estimating rh with heating equation development the magnus equation for partial pressure of water in air is: for a given relative humidity rh in percent and temperatur e in oc. typical values for a1, b1, and c1 are 17.625, 243.04, and 610.94, respectively. if the air is heated, the partial pressure does not change, and the apparent relative hu midity drops according to: or this can be simplified to: p rh 100 --------- - ?? ?? c 1 e a 1 t b 1 t + ?? --------------------- ? ? = p rh? 100 ---------- ?? ?? c 1 e a 1 t ? t + ?? b 1 t ? t ++ ?? ----------------------------------- ? ? = rh? rh e a 1 t ? b 1 t + ?? --------------------- e a 1 t ? t + ?? b 1 t ? t ++ ?? ----------------------------------- ? ----------------------------------------- = rh? rhe a 1 ? b 1 ?? t ? b 1 t + ?? b 1 t ? t ++ ?? ?? -------------------------------------------------------------- = AN607 30 rev. 1.7 linearization the above equation is still too complex to be useful in simple systems; however, it can be noted that, over a narrow temperature range, the relative humidity error is fairly linea r with relative humidity for a given amount of heating and ambient temperature. the error is linear with rh and increases by about 5% rh per c (thi s is the familiar ?mar k lawrence rule of thumb?). thus, the actual rh can be estimated fairly accurately by: the accuracy of this estimate can be improved by meas uring the temperature of the rh sensor and correcting for heating to get the ambient temperature. for 5 c heating, the correction factor varies fr om 0.0598 at 0 c to 0.0435 at 50 c ambient or and finally, ? ? t ? ?? --------------------------------- = t ambient t measured ? t ? = cf 0.0598 0.000346 ? t ambient ? = rh rh measured 1cf ? ? t ? ?? ------------------------------------ = AN607 rev. 1.7 31 a ppendix c?t erm , u nit , and c oefficient r eference ?? absolute vapor pressure? a measure of the actual amount of water present in the air. ?? dew point? for a given rh and temperature, the temperature at which condensation would form if the air were cooled; meaningful as an indicator of comfort. ?? hydrophobic? water repellent/resistant. ?? ip rating? ingress protection rating; first digit indicates le vel of protection against particle; the second digit represents level of protection against liquids. ?? ip67? an ingress protection rating indicating that the as sembly is dust tight (6) and can withstand up to 1 m of water pressure (7). ?? kapton? a polyimide film developed by dupont that is st able over a wide temperature range (up to +400 c). it is available in sheet, tape, and ?dot? form and is used to protect selected components during solder reflow. ?? oleophobic? oil repellent/resistant. ?? relative humidity? absolute_vapor_pressure saturated_vapor_pressure; expressed as a percentage. ?? saturated vapor pressure? the maximum amount of water that the air can hold; dependent on temperature. temperature conversions: table 4. common pressure unit conversions divide to convert unit pascal (pa) hectopascal (hpa) kilopascal (kpa) bar (bar) atmosphere (atm) torr* (torr) lbf/in 2 (psi) multiply to convert pa 1 1x10 ?2 1x10 ?3 1x10 ?5 9.8692x10 ?6 7.5006 x 10 ?3 145.04 x 10 ?6 hpa 100 1 1x10 ?1 1x10 ?3 9.8692x10 ?4 7.5006 x 10 ?1 145.04 x 10 ?4 kpa 1000 10 1 1x10 ?2 9.8692x10 ?3 7.5006 145.04 x 10 ?3 bar 100,000 1000 100 1 0.98692 750.06 14.50377 atm 101,325 1013.25 101.325 1.01325 1 760 14.696 torr 133.322 1.33322 133.322 x 10 ?3 1.3332 x 10 ?3 1.3158 x 10 ?3 1 19.337 x 10 ?3 psi 6.895 x 10 3 68.95 6.895 68.948 x 10 ?3 68.046 x 10 ?3 51.715 1 *note: 1 torr ~= 1 mmhg t c 5 9 -- - t f 32 ? ?? = t f 9 5 -- - ?? ?? t c 32 t k t c 273.15 t r t f 459.69 += += += AN607 32 rev. 1.7 table 5. humidity terms and definitions term definition units p pure water vapor pressure (no air or other gas) n/m 2 or pa p? actual vapor pressure (water vapor in air) n/m 2 or pa p s pure water saturated vapor pressure (no air or other gas) n/m 2 or pa p s ? actual saturated vapor pressure (water vapor in air) n/m 2 or pa p total atmospheric pressure n/m 2 or pa f water vapor enhancement factor dimensionless d v volumetric humidity mass of water vapor/volume of humid gas kg/m 3 m g molal mass (molecular weight) of dry gas (air) kg/mol m v molal mass (molecular weight) of water vapor kg/mol mmass kg n amount of substance in moles mol r universal gas constant in joules per mole of air per k j/(mol)(k) r? universal gas constant in joules per kilogram of air per k j/(kg)(k) s percent of saturation % ? % relative humidity (%rh or %rh) % y mixing ratio kg vapor/kg dry gas kg/kg yw specific humidity kg vapor/kg humid gas kg/kg ? h enthalpy change between phases h ig ice ?? gas h if ice ?? liquid h fg liquid ?? gas ? s entropy change between phases t dp dewpoint temperature AN607 rev. 1.7 33 a ppendix d?n onlinear c orrection of v oltage i nputs with the si7013 the si7013 has the capability to apply a lookup-tab le-based non-linear correction to voltage measurements. this correction is invoked by writing a ?1? to bit 5 of user register 1. note that humidi ty measurements should not be performed when this bit is set. in the discussion below, ?input? refers to the a/d voltage measurement result, which is a 16-bit signed integer, and ?output? refers the output after the non-linear correction, which is assumed to be a 16-bit unsigned integer. the non-linear correction is based on a 10-point table lookup linearization. each point consists of the ideal output for a given expected a/d measurement result. table 6 is st ored in the si7013 memory, which must also have the slope at points 1?9. slope is multiplied by a scaler of 256. only nine of the input/output pairs nee d to be in the table because the 10th pair is determined by the slope equation. overall, the si7013 has 27 16-bit numbers in its table (54 bytes). this table is stored in non-volatile memory of the si7013 and must be programmed based on the desired look-up table. the actual output is determined by extrapolation: if in >in2, out = out1+slope1 x (in-in1)/256 else if in >in3, out = out2+slope2 x (in-in2)/256 else if in >in4, out = out3+slope3 x (in-in3)/256 else if in >in5, out = out4+slope4 x (in-in4)/256 else if in >i n6, out = out5+slope5 x (in-in5)/256 else if in >in7, out = out6+slope6 x (in-in6)/256 else if in >in8, out = out7+slope7 x (in-in7)/256 else if in >in9, out = out8+slope8 x (in-in8)/256 else out = out9+slope9 x (in-in9)/256 slopen 256 outputn 1 outputn ?+ ?? inputn 1 inputn ?+ ?? ------------------------------------------------------------------ ? = AN607 34 rev. 1.7 the table must be arranged in order of decreasing input va lues. the table is entered into memory addresses 0x82? 0xb7 one byte at a time. the table itself is user-programmed, and, by default, all table values are 0xff. it should be noted that, once the non-linear correction data is saved to memory, it cannot be overwritten. as an aid to calculation of the table, several tools have been developed. ?? a spreadsheet, ?si7013 thermistor correction calc ulator.xlsx?, is available under the miscellaneous section of the sensors documentation page at http://www.silabs.com/s upport/pages/document- library.aspx?p=sensors. this spreadsheet calculates the expected output of the a/d based on an assumed thermistor and biasing circuit (the ncp18xh 103f03rb thermistor used on si7013 evaluation boards with 24.3 k ? biasing resistors). then, based on the desir ed output after linearization (in this case, output = (temperature + 46.85) x 65536/175.72), the slope is calculated. finally, the spreadsheet calculates the hexadecimal values that should go in memory locati ons 0x82?0xb7 based on the input/ output and slope values. ?? the si7013 evaluation board has the option of trying di fferent values of linearization based on numbers entered in a gui. these values can be saved to file or burned into the si7013 memory. for example: for the si7013 evaluation board with a 10 k ? thermistor and two 24.3 k ? bias resistors, and assuming the a/d conversion is done using vdd as a reference with buffered inputs, the ideal input volt age version temperature is: vin = vdd x rthemistor/(rthermisor + 46.4 k ? ) since vdd is also the reference, then the expected a/d conversion result is: a/d counts = 32768 x rthemistor/(rthermisor + 46.4 k ? ) if it is desired to linearize this resu lt for the same temperature representation as the onboard temperature sensor: temperature c = (output_code x 175.72/65536 ? 46.85) then, the desired output code is: output_code = 65536 x (temperature + 46.85)/175.72 using thermistor data sheet values of resistance vers us temperature and choosing to linearize at the points (?15 c, ?5 c, 5 c, 15 c, 25 c, 35 c, 45 c, 55 c, 65 c, and 75 c) results in the information in table 7. table 6. memory location descriptions memory location description 0x82 msb of in1 0x83 lsb of in1 ?? 0x93 lsb of in9 0x94 msb of out1 ?? 0xa5 lsb 0f out9 0xa6 msb of slope1 ?? 0xb7 lsb of slope9 AN607 rev. 1.7 35 the values highlighted in gray would be the table entries for the si7013. table 7. example non-linear correction to thermistor voltage measurements thermistor resistance (from data sheet) vin/vdd assuming 24.3 k ? bias resistors a/d codes desired (temperature) code slope 53650 0.524694377 17193 11879 ?256 33892 0.410851961 13463 15608 ?294 22021 0.31181943 10218 19338 ?364 14674 0.231912002 7599 23067 ?476 10000 0.170648464 5592 26797 ?640 6948 0.125081011 4099 30527 ?877 4917 0.091877347 3011 34256 ?1210 3535 0.067804738 2222 37986 ?1684 2586 0.050521627 1655 41715 ?2346 AN607 36 rev. 1.7 entering the table into memory the table is entered into memory addresses 0x82?0xb7, on e byte at a time. for the above example, the values to be written are listed in table 8: table 8. example non-linear thermistor correction entries into si7013 memory table entry hex byte 1 byte 2 memory location table entry hex byte 1 byte 2 memory location table entry hex byte 1 byte 2 memory location 17193 4329 43 82 11879 2e67 2e 94 ?256 ff00 ff a6 29 83 67 95 00 a7 13463 3497 34 84 15608 3cf8 3c 96 ?294 feda fe a8 97 85 f8 97 da a9 10218 27ea 27 86 19338 4b8a 4b 98 ?364 fe94 fe aa ea 87 8a 99 94 ab 7599 1daf 1d 88 23067 5a1b 5a 9a ?476 fe24 fe ac af 89 1b 9b 24 ad 5592 15d8 15 8a 26797 68ad 68 9c ?640 fd80 fd ae d8 8b ad 9d 80 af 4099 1003 10 8c 30527 773f 77 9e ?877 fc93 fc b0 03 8d 3f 9f 93 b1 3011 0bc3 0b 8e 34256 85d0 85 a0 ?1210 fb46 fb b2 c3 8f d0 a1 46 b3 2222 08ae 08 90 37986 9462 94 a2 ?1684 f96c f9 b4 ae 91 62 a3 6c b5 1655 0677 06 92 41715 a2f3 a2 a4 ?2346 f6d6 f6 b6 77 93 f3 a5 d6 b7 AN607 rev. 1.7 37 the command code, 0xc5, is used for programming; so, for example, to program a si7013 at slave address 0x40 with the values above starting with 0x4c to memory location 0x82, one would write: AN607 38 rev. 1.7 a ppendix e?t hermal m odel for a s ensor on a p addle to illustrate some considerations for separating the sensor from the syst em, consider the following practical example of the sensor on a 3 cm x 3 cm pcb connected by a 1 cm wide piece of pcb material 3 cm long: figure 14. sensor on a 3x3 cm pcb referring to the thermal model discussed in ?3.2. temperature and humidity sensor placement? : figure 15. general thermal model for sensor placement 3cm 3cm 1cm 3 ? cm sensor tambient sensor tambient system r3 r2 r1 c1 c2 AN607 rev. 1.7 39 the thermal impedance to air for a standard fr4 pcb is about 1000 c/w per cm 2 of pcb area. with two sides exposed to the ambient a total of 18 cm 2 is connected to the ambient. this makes r1 55.5 c/w. the mass of 9 cm 2 of pcb material is about 2.5 g (the specific gravity of fr4 is 1850 kg/m 3 and assuming 1.5 mm thickness) and the specific heat capa city of pcb material is 0.6 j/(g-c), so the heat capacity is 1.5 j/c. the time constant r1 x c2 is 1.5 j/c x 55.5 c?second/j = 83 seconds. this time constant is independent of the pcb area? more area means lower thermal impedance but higher thermal mass. to improve the time constant beyond this thinner pcb material would have to be used or there would need to be fins or airflow to reduce the thermal impedance. turning our attention to r2 the thermal conductivity of fr 4 material in plane is around 1watt/meter-c. so, for the example of the connection of the sensor area to the rest of the system by 1cm wide 3cm long 1.5mm thick fr4: conductance = thermal conductivit y x area/length = 1 watt/meter- c x 10 ?2 m/cm x 1 x 0.15/3 = 0.05 x 10 ?2 w/ c or thermal impedance (1/conductance) is 2000 c/w (this is r2). this is assuming minimal copper routing on the connector material. with this design since r2 is 36 times r1, so the system heating will have a fa irly minor effect. for example if the system heating is 10 c the sensor temperature would only increase 0.3 c. if more thermal connection is tolerable the connector area could be made shorter or wider or the pcb area the sensor is connector to could be made smaller. this example was intended to illustrate the th ermal design considerations for go od response to ambient conditions and insulation from the system. in some cases, it is not possible to place the sensor in sufficient thermal contact with the environment to shield it for the thermal mass and heat sources in th e system. in this case, it is often possible to compensate for the system by placing an addi tional temperature sensor in the system. however, in all cases, the thermal contact of the sens or to the environment should be maximized, and the thermal contact of the sensor to the rest of the system should be minimized. AN607 40 rev. 1.7 d ocument c hange l ist revision 0.1 to revision 1.0 ? updated storage, handling, and assembly instructions. ? corrected table 4, ?common pressure unit conversions,? on page 31. revision 1.0 to revision 1.1 ? multiple updates to include si7013, si7020, and si7021 parts. revision 1.1 to revision 1.2 ? added "4.2.use of confor mal coating and under-fill materials" on page 11. ? corrected simplified magnus equation on page 27. revision 1.2 to revision 1.3 ? added link to si7013 thermistor correction calculation table on page 31. revision 1.3 to revision 1.4 ? added sections 3.2.2 and 3.2.3. ? updated section 3.3. ? added appendix e. revision 1.4 to revision 1.5 ? revised to include si7006, si7007, si7022, and si7023. revision 1.5 to revision 1.6 ? updated title to include temperature sensor. ? updated to include si7050, si7053, si7054, and si7055. revision 1.6 to revision 1.7 ? added si7034. disclaimer silicon laboratories intends to provide customers with the latest, accurate, and in-depth documentation of all peripherals and modules available for system and software implementers using or intending to use the silicon laboratories products. characterization data, available modules and peripherals, memory sizes and memory addresses refer to each specific device, and "typical" parameters provided can and do vary in different applications. application examples described herein are for illustrative purposes only. silicon laboratories reserves the right to make changes without further notice and limitation to product information, specifications, and descriptions herein, and does not give warranties as to the accuracy or completeness of the included information. silicon laboratories shall have no liability for the consequences of use of the information supplied herein. this document does not imply or express copyright licenses granted hereunder to design or fabricate any integrated circuits. the products must not be used within any life support system without the specific written consent of silicon laboratories. a "life support system" is any product or system intended to support or sustain life and/or health, which, if it fails, can be reasonably expected to result in significant personal injury or death. silicon laboratories products are generally not intended for military applications. silicon laboratories products shall under no circumstances be used in weapons of mass destruction including (but not limited to) nuclear, biological or chemical weapons, or missiles capable of delivering such weapons. trademark information silicon laboratories inc., silicon laboratories, silicon labs, silabs and the silicon labs logo, cmems?, efm, efm32, efr, energy micro, energy micro logo and combinations thereof, "the world?s most energy friendly microcontrollers", ember?, ezlink?, ezmac?, ezradio?, ezradiopro?, dspll?, isomodem ?, precision32?, proslic?, siphy?, usbxpress? and others are trademarks or registered trademarks of silicon laboratories inc. arm, cortex, cortex-m3 and thumb are trademarks or registered trademarks of arm holdings. keil is a registered trademark of arm limited. all other products or brand names mentioned herein are trademarks of their respective holders. http://www.silabs.com silicon laboratories inc. 400 west cesar chavez austin, tx 78701 usa smart. connected. energy-friendly products www.silabs.com/products quality www.silabs.com/quality support and community community.silabs.com |
Price & Availability of AN607
 |
|
|
|
|
All Rights Reserved © IC-ON-LINE 2003 - 2022 |
| [Add Bookmark] [Contact Us] [Link exchange] [Privacy policy] |
|
Mirror Sites : [www.datasheet.hk]
[www.maxim4u.com] [www.ic-on-line.cn]
[www.ic-on-line.com] [www.ic-on-line.net]
[www.alldatasheet.com.cn]
[www.gdcy.com]
[www.gdcy.net] |